RNA editing, the modification of nucleotides within an RNA species, is widespread in mammals. RNA editing has the ability to alter the coding capacity of protein coding transcripts as well as modify the post- transcriptional regulation of targeted transcripts. Recent advancements in high throughput RNA sequencing has substantially increased the number of known targets for RNA editing as well as the tissues in which RNA editing has been observed. Unfortunately, the targets and result of RNA editing is unclear for many systems. The goal of this work is to define the role of RNA editing in male fertility, specifically the development of post- meiotic germ cells. Previous work has demonstrated that global ablation of a putative RNA editing regulator results in male infertility due to defects in post-meiotic germ cells. The long-term goal of this work is to functionally characterize the role of RNA editing in male fertility and determine if abnormal RNA editing is causative to some male infertility. Infertility affects one in seven couples, roughly half of these cases being a result of male factor defects. Of these, nearly half again are idiopathic, with the molecular underpinnings of infertility undefined. Using global and germ-cell specific knockout mouse models of RNA editing enzymes, this proposal will test the hypothesis that germ cell RNA editing results in changes to the germ cell proteome and is fundamental for male fertility. Aim 1 is to determine whether global loss of the previously described putative RNA editing regulator changes the frequency and/or site distribution of RNA editing in the testis. Aim 2 will establish if there is a requirement for a key RNA editing enzyme in germ cell development and fertility and determine if germ-cell specific loss of the enzyme alters testicular RNA editing. Lastly, Aim 3 will determine whether RNA editing in mRNA coding regions impacts the protein coding capacity or translation of edited targets by mass spectrometry detection of peptides coded for by edited transcripts and the polysomal association of edited transcripts relative to their unedited counterparts. The knowledge gained from these studies represents a substantial contribution to the fields of male reproduction and RNA editing and may provide insight into the mechanisms of idiopathic male infertility.